Ovarian Steroids Modulate the Self-Priming Effect of Luteinizing Hormone-Releasing Hormone on Bovine Pituitary Cells in Vitro*

Abstract

LHRH has the ability to prime the anterior pituitary gland, thereby increasing the quantity of LH and FSH released by a standard dose of LHRH. This priming effect may play an important regulatory role in LH secretion. In experiments described herein, we demonstrate that LHRH priming occurs via a direct effect on bovine gonadotrophs and LHRH priming is modulated by ovarian steroids. Bovine pituitary cells were primed as follows: Medium was changed at time zero, then at 50-min intervals for 400 min. LHRH was added 10 min before each medium change, i.e. cells were exposed to LHRH for 10 min during each of eight consecutive 50-min incubations. At the end of this 400 min, the degree of priming was assessed by incubating the pituitary cells with 0 or 1 ng/ml LHRH for 4 h (test period). The first variable tested was priming dose of LHRH (0, 0.001, 0.01,0.1, or 1 ng/ml). In general, LHRH priming did not affect basal release of LH but did increase the ability of a test dose of LHRH to induce LH release. For example, basal LH release during the 4-h test period in control cultures averaged 13 ± 0.5 ng/ml and was not affected by priming with 0.001, 0.01, or 0.1 ng/ml LHRH. But, basal LH release was increased to 45 ± 3 ng/ml when primed with 1 ng/ml LHRH. Priming with 0.001, 0.01, and 0.1 ng/ml LHRH increased LHRH-induced release of LH 46%, 77%, and 47%, respectively. To determine if ovarian steroids modulate the ability of LHRH to prime, bovine pituitary cells were incubated with estradiol (E₂; 0.5 ng/ml) and progesterone (P4; 3.1 ng/ml) alone or in combination. The design was a 4 × 2 × 2 factorial with main effects, steroid treatment (no steroid, E₂, P₄, and E₂ plus P₄), priming concentration of LHRH (0 and 0.01 ng/ml), and test dose of LHRH (0 and 1 ng/ml). Steroids were present throughout the priming and test periods. The increase in LHRH-induced LH release attributed to priming averaged 21.6 ng/ml in cultures incubated in the absence of supplemental steroids and was increased to 48.3 ng/ml by E₂. But, P₄ alone and P₄ plus E₂ completely inhibited this priming effect. We conclude that LHRH can prime bovine pituitary cells by a direct effect on the gonadotrophs, and the degree to which priming occurs is dependent on the ovarian steroid milieu.